Wet air oxidation is a well-known treatment process for the removal of COD and BOD from industrial and municipal wastewater streams. The processes involve contacting a wastewater with an oxidizing source, such as oxygen, ammonium nitrate and nitric acid at elevated temperatures and pressures to oxidize pollutants. Most carbonaceous material is converted to carbon dioxide. The nitrogen present either from organo-nitrogen compounds or other sources are converted to nitrogen.
The following references illustrate wet oxidation processes:
Proesmans, Luan and Buelow of Los Alamos National Laboratory (Ind. Eng. Chem. Res. 1997, 36 1559-1566) report on a high temperature and pressure (500xc2x0 C./345 bar) hydrothermal oxidation process to remove organic compounds from a waste stream using ammonium nitrate as the oxidizing agent. In the oxidation of methanol and phenol, the authors report that unless an excess of oxidizable carbon is present, NOx in the effluent may become a problem. To avoid NOx production and reduce carbon components to carbon dioxide, a polishing step using hydrogen peroxide is suggested.
GB 1,375,259 discloses the wet oxidation of carbon and nitrogen containing materials to gaseous reaction products using HNO3 and/or a nitrate as oxidizing agent, at temperatures of between 150xc2x0 C. and the critical temperature of water. The preferred oxidizing agent is NH4NO3, which disappears completely from the reaction medium. Example VII shows the treating of a waste stream of caprolactam, the sodium salt of aminocaproic acid and sodium sulfate with nitric acid at a temperature of 300xc2x0 C. at 15 bars. The patentees report that slow heating of the reaction mixture resulted in reduced corrosiveness of the reactant mixture.
U.S. Pat. No. 4,654,149 discloses the use of a noble metal catalyst supported on a titania carrier in a wet oxidation process to decompose ammonium nitrate at 250xc2x0 C. for 60 minutes. Approximately from 50-99% decomposition of both ammonium nitrate and nitrite is achieved without air present. Further examples show wet oxidation of phenol with 0.2 times the required amount of oxygen.
JP 60-98297, JP 61 257,292 and JP 61 257,291, discloses the catalytic wet oxidation of ammonium nitrate wastewaters with 1.0 to 1.5 times the stoichiometric oxygen required for ammonia decomposition, at a pH of 3-11.5 at a temperature from 100 to 370xc2x0 C. with a supported noble metal catalyst.
U.S. Pat. No. 5,118,447 discloses a process for the thermochemical nitrate destruction where an aqueous solution of nitrate or nitrite is contacted with a stoichiometric amount of formic acid or formate salt, depending upon the pH. Wet oxidation is effected by heating to 200 to 600xc2x0 C. in the liquid phase to form elemental nitrogen and carbon dioxide. The reaction may be carried out over a pH range of 0-14.
U.S. Pat. No. 5,221,486 discloses a denitrification process where the types of nitrogen compounds present in a waste stream are identified and quantified. The oxidized and reduced forms of nitrogen are balanced and, then, an appropriate nitrogen containing reactant, such as ammonia or a nitrite or nitrate compound, is added and the mixture is heated to 300 to 600xc2x0 C. under pressure to effect denitrification.
U.S. Pat. No. 5,641,413 discloses the two stage wet oxidation of wastewater containing a carbonaceous and nitrogen species. In the first stage the COD is removed by wet oxidation at a temperature of less than 373xc2x0 C. and a pressure sufficient to maintain a liquid water phase. The remaining nitrogen compounds are converted to nitrogen in the second stage by adding sufficient inorganic nitrogen-containing compound to the oxidized wastewater to produce essentially equal concentrations of ammonia-nitrogen, nitrite-nitrogen plus nitrate-nitrogen and a waste stream of reduced COD. Mineral acid is added to the oxidized wastewater to produce a pH between 4 and 7. Optionally, a transition metal salt is added, to catalyze a thermal denitrification step. The last step is conducted at 100xc2x0 to 300xc2x0 C. to decompose the nitrogen compounds.
D. Leavitt et al in Environmental Progress 9 (4), 222-228 (1990) and in Environ. Sci. Technol. 24 (4), 566-571 (1990) reported that 2,4-dichlorophenoxyacetic acid, atrazine and biphenyl were converted to CO2 and other non-harmful gases (N2 and N2O) trough the homogeneous liquid phase oxidation with ammonium nitrate. These reactions were carried out by dissolving the substrates in polyphosphoric acid, adding ammonium nitrate and then heating to about 260xc2x0 C. for some period of time. Although this clearly shows that ammonium nitrate is a good oxidizing agent, it is not a process lending itself to treating aqueous waste streams containing only 1,000 to 10,000 ppm TOC.
This invention relates to an improvement in the wet oxidation of waste streams using ammonium nitrate as the oxidizing agent. The basic wet oxidation process comprises adding ammonium nitrate or precursors thereof to a waste stream in desired amount to reduce the carbonaceous components to carbon dioxide and the nitrogen components to nitrogen. In our co-pending application there was proposed an improvement for reducing the corrosiveness of waste streams contaminated with sulfur or phosphorous containing compounds, whether organic or inorganic, while maintaining reaction rate. The process comprised: operating said process within a pH from about 1.5 to 8 and preferably within a pH range of from about 1.8 to 4. The improvement residing herein comprises adding organic material to the waste stream to provide acetate ion in a molar ratio from 0.06 to 0.17 moles per mole nitrate or, in the alternative, should the waste stream contain organic material convertible to acetate in the wet oxidation process, maintain a level of organic material sufficient to provide acetate ion in an amount of at least 0.06 moles per mole of nitrate. The addition, or maintenance of organic material convertible to acetate ion acts as a corrosion inhibitor or buffer assisting in reducing corrosion at pH values of 4 and lower.
The process of this invention offers several advantages and they include:
an ability to minimize the corrosiveness of wet air oxidation streams when operating at a low pH, and.
an ability to maintain excellent reaction rates.
This invention relates to an improvement in wet oxidation processes involving the destruction of carbonaceous components and nitrogenous components in industrial and municipal wastewater contaminated with sulfur or phosphorus containing components or salts of weak acids and strong bases. The process is a single step wet oxidation process that employs ammonium nitrate or precursor thereof as the oxidizing agent and material convertible to acetate ion. Oxygen gas is not required. The process operates in a pH region between 1.5 and about 8 and preferably within a pH region of from about 1.8-4.
The first step in accomplishing removal of carbonaceous and nitrogenous components to a desirable level requires balancing the oxidation and reduction properties of all of the oxidizable and reducible species present in the wastewater stream. All nitrogen containing species, organic or inorganic, produce substantially only nitrogen and minor amounts of nitrous oxide gas and all carbon containing species produce substantially only carbon dioxide.
One key to pH control in the first step, and to the maintaining of reaction rate during wet oxidation of wastewater streams contaminated with sulfur or phosphorus substances and alkali and alkaline earth metals (designated M), is in the control of the M/SO4xe2x88x922 and M/PO4xe2x88x923 ratio (equivalence basis). This is accomplished as follows: contaminants whose anions are of strong acids, e.g., sulfate and phosphates are balanced with alkali or alkaline earth metal: cations and conversely, cations of strong bases are balanced with sulfate or phosphate. The ratio of M/SO4xe2x88x922 is maintained from 0.1 to 4, preferably 0.2 to 1, most preferably from 0.4 to 0.7 and the ratio of M/PO4xe2x88x923 of from 0.1 to 2, preferably 0.2 to 0.67 during wet oxidation. Lower ratios,  less than 0.4 for M/SO4xe2x88x922 may be tolerated when the process effluent designed permits operation with some residual carbon compounds in the effluent. High ratios reduce reaction rate.
The second step of the process involves the balancing of organic species such that on substantial reduction of nitrogen in the wet oxidation process there remains sufficient water soluble, (0.6, preferably  greater than 7 g per 100 g water) acetate or carbonaceous material, convertible to acetate ion, to aid in reducing corrosion particularly when the pH is below 4 and more particularly below 2. This is accomplished by providing or preventing the molar ratio acetate to nitrate from falling below 0.06:1.
The second step in the process involves the balancing of organic species such that on substantial reduction of nitrogen in the wet oxidation process there remains sufficient carbonaceous material in solution under the process conditions in the form of e.g., a) acetic acid and/or its derivatives such as esters, amides, salts, etc; or b) carbonaceous compounds that upon oxidation are precursors to acetic acid or its derivatives.
More specifically, the improved method for pH control In the first step and described in our co-pending application comprises maintaining the ratio of M/SO4xe2x88x922 of from 0.1 to 4, preferably 0.2 to 1.0 and most preferably a ratio of 0.4 to 0.7. A ratio of M/PO4xe2x88x923 of from 0.1 to 2, preferably 0.2 to 0.67 during the wet oxidation process is used. M is an alkali metal or alkaline earth metal cation and the ratio of M/SO4xe2x88x922 and M/PO4xe2x88x923 is. based upon an equivalence basis. By maintaining these ratios, while balancing the reaction such that the carbon species is converted to carbon dioxide and the nitrogen containing species is converted to nitrogen, the corrosiveness of the reaction mixture is reduced and the reaction rate is maintained.
To implement the first step and effect balancing of the components in the waste stream, the waste stream is analyzed for composition using well-known analytical procedures, e.g., ion, gas and liquid chromatography and ICP-AES. First, the carbon content in terms of COD and TOC is determined, particularly if the organic components are difficult to analyze on a component-by-component basis. Ascertaining the quantity of COD test can be accomplished by oxidizing a known volume of the wastewater with potassium dichromate and expressing the result as the weight of oxygen required to oxidize the carbon in the sample to carbon dioxide. Since the COD measures the mg of O2 required to oxidize 1 liter of wastewater containing the reducible species, one only needs to equate this number to the equivalent weight of ammonium nitrate needed to do the same job. There are some cases where a COD measurement will not adequately represent the total amount of reducibles. Certain amines and refractory organics are not readily oxidized by dichromate and thus are not accounted for by the COD measurement. However even if this type of organic is present, the COD measured is a good starting point for determining the amount of ammonium nitrate needed for treatment. If some of the TOC is not oxidized, then the amount of ammonium nitrate added to the influent may be adjusted (an iterative process) until enough is present to oxidize the reducibles to the desired level. Sometimes one may want to operate to completion and sometimes less than completion depending upon the desired effluent specifications.
Once the above analytical analysis of the wastewater is performed, the reduction/oxidation (redox) half reactions for the wet oxidation process can be written. This requires an identification of the oxidizing species and the reducing species. For simplification, the following guidelines may be used:
Those carbon containing species, including those with heteroatoms, where the carbon atom is oxidized on conversion to carbon dioxide are reducing species or agents. Carbon dioxide, bicarbonates, carbonates and the like, which maintain the same oxidation state are not.
Those nitrogen containing species where the nitrogen atom is oxidized on conversion to nitrogen are also reducing species. The ammonium ion is a reducing species.
Those nitrogen containing species where the nitrogen atom is reduced on conversion to nitrogen are oxidizing species. The nitrate and nitrite ions are oxidizing species.
Oxygen gas and peroxy oxygen are oxidizing species.
Any sulfur atom in any organic or inorganic species will change to sulfate ion
Any phosphorus atom in any organic or inorganic species will change to phosphate ion.
Any oxygen atom in any organic or inorganic peroxide species will change to Oxe2x88x922, as in water.
The following elements are assumed that the oxidation/reduction state does not change during wet oxidation:
Organo halogen or halide ion
Ether, alcohol and carbonyl oxygen
Alkali and alkaline earth metal cations.
Common metal cations (e.g., generated from materials of construction) in their highest normal oxidation state, e.g., Fe+3, Ni+2, Cr+3, Al+3, Cu+2, Zn+2, etc.
The oxidizing species in the wet oxidation process is ammonium nitrate and precursors thereof. In the process, ammonium nitrate may be added to the wastewater in the desired amount or ammonium nitrate may be added by introducing ammonia or nitric acid, depending upon the extent of the contaminant in the wastewater stream.
The maintenance of pH of the reaction stream is based upon controlling the M/SO4xe2x88x922 and M/PO4xe2x88x923 ratio by addition of alkali metal and alkaline earth metals or sulfuric acid or phosphoric acid to the wastewater as required. When the M/SO4xe2x88x922 ratio or the M/PO4xe2x88x923 ratio is calculated to fall below specified limits, e.g., preferably not below 0.2, alkali is added. When the ratio exceeds about 1.0 for M/SO4xe2x88x922, preferably 0.67 for M/PO4xe2x88x923, sulfate or phosphate are added as ammonium salts or free acids to reduce the pH. A high M/SO4xe2x88x922 and M/PO4xe2x88x923 ratio can lead to excessive reaction times.
The alkali and alkaline earth metal source for the wet oxidation is typically a sodium ion although other alkali metals such as lithium, potassium, cesium and so forth may be used. For reasons of efficiency, the alkali metals are added as the oxide or hydroxide. If substantial levels of alkali metal salts of organic substances are added, then the anionic component must be considered in the redox half reaction. When the anion is an oxide or the hydroxide, the anion need not be considered. Alkaline earth metals may be used in the wet oxidation process to provide the M component. However, these metals often form insoluble salts and thus may cause plugging or fouling the reactor. Therefore, in the practice of the process, it is preferred to use Group 1 metals, preferably sodium, to provide the cationic component M.
The second step of the improved wet oxidation process involves analyzing the waste stream to determine the organic content and to determine those components oxidizable to acetate ion. If none exists, acetate must be added to the waste stream to provide the necessary level of acetate corrosion inhibitor, i.e., 0.06 to 0.17 moles per mole of nitrate ion. If organic material convertible to acetate is present, then it is necessary that conditions exist to provide at least 0.06 moles acetate ion per mole of nitrate. Although the alkaline metal addition in the first step is necessary to maintain pH control, there is some corrosion. The membrane of acetate ion in the wet oxidation process reduces corrosion even further at the lower pH.
Acetate providing material includes all forms of acetate, e.g. alkali, alkaline earth and ammonium acetate; alkyl substituted aromatics and the like.
This wet oxidation process operates generally without the use of a catalyst at temperatures near the supercritical temperature of water, i.e., 300 to 400xc2x0 C., preferably 360 to 374xc2x0 C. Transition metal catalysts such as those used in wet oxidation processes can be added as need or desired. These metals include V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Mo.
Pressures are controlled to a high enough pressure to maintain a liquid phase behavior for both the influent and the effluent. If gas phase conditions occur, the salts in the wastewater oxidation product may precipitate and cause plugging of the reactor.